LM4817 datasheet(17/21 Pages) NSC

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Application Information (Continued)

the error amplifier. The second term expresses the zero

(numerator) and pole (denominator) of the load in combina-

tion with the R

O of the regulator.

Figure 5 shows a Bode plot that represents a case where the

zero contributed by the load is too high to cancel the effect of

the pole contributed by the load and R

O. The solid line

illustrates the loop gain while the dashed line illustrates the

corresponding phase shift. Notice that the phase shift at

unity gain is a total 360˚ -the criteria for oscillation.

The LDO generates an internal zero that makes up for the

inadequately high zero of the low ESR ceramic output ca-

pacitor. This internally generated zero is strategically placed

to provide positive phase shift near unity gain, thus providing

a stable phase margin.

No-Load Stability

The LM4817 remains stable during no-load conditions, a

necessary feature for CMOS RAM keep-alive applications.

Input Capacitor

The LM4817 requires a minimum input capacitance of about

1µF. The value may be increased indefinitely. The type is not

critical to stability. However, instability may occur with bench

set-ups where long supply leads are used, particularly at

near dropout and high current conditions. This is attributed to

the lead inductance coupling to the output through the gate

oxide of the pass transistor; thus, forming a pseudo LCR

network within the Loop-gain. A 10µF tantalum input capaci-

tor remedies this non-situ condition; its larger ESR acts to

dampen the pseudo LCR network. This may only be neces-

sary for some bench setups. 1µF ceramic input capacitor are

fine for most end-use applications.

If a tantalum input capacitor is intended for the final applica-

tion, it is important to consider their tendency to fail in short

circuit mode, thus potentially damaging the part.

Noise Bypass Capacitor

The noise bypass capacitor (CC) significantly reduces the

LDO’soutput noise. Connect the CC capacitor between pin 6

and ground. The optimum value for CC is 33nF.

Pin 6 directly connects to the high impedance output of the

bandgap. The DC leakage of the CC capacitor should be

considered; loading down the reference will reduce the out-

put voltage. NPO and COG ceramic capacitors typically offer

very low leakage. Polypropylene and polycarbonate film car-

bonate capacitor offer even lower leakage currents.

CC does not affect the transient response; however, it does

affect turn-on time. The smaller the CC value, the quicker the

turn-on time.

200781B8

FIGURE 5. Loop Gain Bode Plot Illustrating

Inadequately High Zero for Stability Compensation

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